Structural phase transition behavior of Pr2NiO4+delta, which attracts interest as new cathode material for solid oxide fuel cells, was investigated by DSC and TG-DTA under controlled oxygen partial pressure, P(O-2). It was revealed that the structural phase transition from orthorhombic phase to tetragonal phase involved discrete decrease of oxygen content in Pr2NiO4+delta and that the phase transition temperature, T-p, decreased with decreasing P(O-2). Variation of enthalpy, Delta H, and entropy, Delta S, at the phase transition per one molar Pr2NiO4+delta were calculated from the peak area of DSC and they were independent on P(O-2) between 1.0 x 10(-1) bar and 2.25 x 10(-3) bar. From the linear relationships between T-p and RTpIn P(O-2) and between 1/T-p and RIn P(O-2), variation of standard enthalpy, Delta H degrees, and standard entropy, Delta S degrees, of the phase transition per one molar O-2 were calculated, showing fair agreement with Delta H and Delta S. The variation of oxygen content at the phase transition, Delta delta, increased with decreasing P(O-2), which can be attributed to valence variation of Pr in tetragonal Pr2NiO4+delta by P(O-2). (C) 2013 Elsevier B.V. All rights reserved.